This invention relates to a friction control element, such as a hydraulically or pneumatically actuated clutch, of the type used to control operation of an automatic transmission. In particular, the invention pertains to a control for producing staged engagement and disengagement of such a clutch having at least two sealed areas on its actuating piston.
Automatic transmissions are typically designed to transmit full engine torque and the engine torque as amplified by a torque converter at stall torque under static. i.e., non-shifting conditions. The control system of an automatic transmission includes a low/reverse clutch, which is applied or engaged to produce the lowest forward speed ratio and the reverse drive speed ratio. Such engagement produces a drive connection between components of the planetary gearing, which when selectively combined with the engagement of other control elements, results in the transmission operating in low gear or reverse gear. When the clutch is disengaged, another of the several forward speed ratios can be produced upon engagement of another combination of friction control elements. Therefore, gearshifts into and out of low gear, 1–2 upshifts and 2–1 downshifts, are produced at least in part by engaging and disengaging, respectively, the low/reverse clutch. Throughout this discussion, the term “friction control element” refers to a hydraulically actuated friction clutch or brake of a control system.
In order for the transmission to have the static torque capacity required to hold full stall torque, the low/reverse clutch is typically designed with a high gain to provide the required torque capacity to the low/reverse clutch. This high gain requirement, however, can affect good shift quality.
In a fully synchronous automatic transmission, all the gear ratio changes occur by coordinating the simultaneous disengagement and engagement of two friction control elements. In a fully synchronous automatic transmission, the low/reverse clutch controls 2–1 downshift events using a low gain clutch. In order to meet the shift quality requirements for all 2–1 events as well as to provide the static capacity required to hold stall torque, a low/reverse clutch must have at least two magnitudes of gain. A clutch having only a single gain will not suffice.
A clutch can produce multiple gains by providing multiple pressure areas on the hydraulic piston that actuates the clutch, primary and secondary pressurized areas. Production automatic transmissions have used this design technique in combination with control of the secondary pressure area on the actuating piston through operation of the transmission manual valve. This approach merely pressurizes both piston areas based on manual valve position with some degree of hydraulic control.
There is a need to provide direct control of the secondary area, preferably under control of an electronic control module and a pressure control device. This need is especially acute for a synchronous transmissions.
This invention provides direct control when the secondary area applies and is controlled via the electronic control module and a pressure control device. This allows the dual area clutch design to be used for shift events in synchronous transmissions.